Use stdbool.h if suitable

Using the standard bool type provided by C allows some recent compilers
and debuggers to give better diagnostics.  Also, some extension code and
third-party headers are increasingly pulling in stdbool.h, so it's
probably saner if everyone uses the same definition.

But PostgreSQL code is not prepared to handle bool of a size other than
1, so we keep our own old definition if we encounter a stdbool.h with a
bool of a different size.  (Among current build farm members, this only
applies to old macOS versions on PowerPC.)

To check that the used bool is of the right size, add a static
assertions about size of GinTernaryValue vs bool.  This is currently the
only place that assumes that bool and char are of the same size.

Discussion: https://www.postgresql.org/message-id/flat/3a0fe7e1-5ed1-414b-9230-53bbc0ed1f49@2ndquadrant.com

src/pl/plperl/plperl.h

@@ -50,6 +50,11 @@
 #define __inline__ inline
 #endif
 
+/*
+ * Prevent perl from redefining "bool".
+ */
+#define HAS_BOOL 1
+
 
 /*
  * Get the basic Perl API.  We use PERL_NO_GET_CONTEXT mode so that our code
@@ -91,11 +96,6 @@
 #define NEED_sv_2pv_flags
 #include "ppport.h"
 
-/* perl may have a different width of "bool", don't buy it */
-#ifdef bool
-#undef bool
-#endif
-
 /* supply HeUTF8 if it's missing - ppport.h doesn't supply it, unfortunately */
 #ifndef HeUTF8
 #define HeUTF8(he)			   ((HeKLEN(he) == HEf_SVKEY) ?			   \